Round-Trip Efficiency Explained: How Much Energy Does a Home Battery Give Back?
- Round-trip efficiency is energy out divided by energy in.
- Losses happen in the battery cells, the inverter and standby electronics.
- Datasheet figures depend on test conditions and what is included.
- Lower efficiency means you need a bigger price gap for storage to pay.
- It is one factor among several, alongside tariff, usage and battery wear.
Round-trip efficiency is the share of the electricity you put into a battery that you get back out. No battery returns everything, so the figure sets a floor under how wide the price gap between cheap and expensive electricity needs to be.
What the number means
If you charge a battery with 10 kWh and later draw 9 kWh from it, the round-trip efficiency is 90%. The missing 1 kWh is mostly lost as heat. These numbers show the arithmetic only. They are not a claim about any product.
The word 'round-trip' matters. It counts the whole journey in and out, not just one direction.
Where the losses happen
Energy is lost at several points, and the total depends on how the system is built:
- Battery cells: charging and discharging generates heat, more so at high power and in cold conditions.
- Inverter: batteries store direct current, while your home runs on alternating current. Each conversion loses a little. The number of conversions depends on whether the battery is AC-coupled or DC-coupled to your solar panels.
- Standby use: control electronics, communications and sometimes cooling draw power even when the battery is idle.
We do not quote typical percentages here. Published figures vary by test method and we have not tested these products ourselves.
Why datasheet figures can mislead
Manufacturers may state efficiency for the battery alone or for the whole system, at a particular power level and temperature, and some quote a direct-current figure that leaves out the inverter. A number that excludes the inverter will look better than what you see at your sockets.
When you compare products, ask what the figure includes, at what charge and discharge power it was measured, and whether standby use is counted. Our Compare For Yourself tool helps you line up datasheet figures side by side.
Good to know
Efficiency losses are a cost of every cycle, so a battery that cycles more often loses more energy in total, even if each cycle looks efficient.
What it means for your savings
Here is the simple break-even maths. If you charge at an import price p and the round-trip efficiency is e, each kWh you get back effectively cost p divided by e. At 90% that is about 1.11 times p. The price you avoid by using stored energy, or the export payment you give up, has to beat that for the cycle to make sense.
The same applies on solar. A kWh stored instead of exported only helps if what you avoid buying later is worth more than the export payment you gave up, after losses. How much you gain overall depends on your tariff, your usage and how the battery is controlled, which we cover in how a home battery saves money and AI smart charging explained.
Common questions
What is a good round-trip efficiency?
Published figures vary, and we do not quote a benchmark. Compare like with like: same test method, same power level, same scope of what is included.
Does efficiency change as the battery ages?
A battery's capacity declines with use, which is why warranties exist. We could not find a general figure for how efficiency changes, so ask the manufacturer for ageing data.
Is standby consumption part of round-trip efficiency?
It depends on the manufacturer. Some include it and some list it separately. Ask for the idle power draw in watts.
Does a DC-coupled battery have better efficiency?
It can involve fewer conversions when charging from solar, but real results depend on the product. Check the datasheet and ask the installer for system-level figures.
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